1. Field of the Invention
This invention relates to image processing performed in an image forming apparatus such as a multifunction printer (MFP). In particular, this invention relates to data compression processing using a method of limiting the number of colors employed in a certain area in order to reduce an amount of data (block approximation encoding).
2. Description of the Related Art
Recently, in various types of image forming apparatuses, the amount of data processed therein has been significantly increasing with the popularization of color images, an increase in the number of gradations, and the like. For overcoming this, various data compression techniques have been developed. One of the techniques is described, which is block approximation encoding for image compression. Specifically, a multi-gradation image is divided into pixel areas each containing a plurality of adjacent pixels. Then, each of the pixel areas is represented by a representative color of pixels the number of which is lower than the number of pixels within the area and pattern information indicative of locations of the representative color pixels. (See Japanese Patent Laid-Open No. H9-327015 (1997))
(First Problem)
The conventional block approximation encoding disclosed in Japanese Patent Laid-Open No. H9-327015 (1997) is a compression method that is effective for maintaining the high quality of images in regard to image data in which the number of used colors is limited to some extent, for example, print image data such as PDL data or computer graphics.
However, a reduction in the number of colors may possibly cause noticeable degradation in quality of an image of multi-gradation image data entered from an input device such as a scanner or a digital camera.
(Second Problem)
Under present circumstances, in the case of JPEG compression employed in a method for compressing multilevel image data, performing the compression and decompression processing to an edge of a letter in an image significantly causes degradation in image, resulting in noticeable degradation in image quality. The stronger the edge component, the more noticeable the degradation in image quality becomes.
For this reason, when the conventional block approximation encoding is employed for the compression and decompression processing, the edge component in the image uncompressed is made stronger, and JPEG processing makes the degradation of the edge of a letter more noticeable.
The present invention is made in view of the foregoing problems and an object of the present invention is to provide an image processing device and an image processing method which are capable of compressing data with minimization of image degradation even if the data is image data entered from an input device such as a scanner.